This application claims the priority of Korean Patent Application No. 10-2002-0088039, filed on Dec. 31, 2002 in the Korean Intellectual Property Office, and PCT/KR2003/002939, filed Dec. 31, 2003, the disclosures of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a molding system having a micro heating element and a method for fabricating a mold insert used therein. More particularly, the present invention relates to the molding system for molding a micro pattern structure having the micro heating element which includes a micro heater formed in an integrated manner by using a MEMS (MicroElectroMechanical Systems) process, and the method for fabricating the mold insert for molding the micro pattern structure used therein.
2. Description of the Related Art
Now, an injection molding, an injection compression molding, a compression molding and a hot embossing molding are being used for forming a high-performance plastic product having a micro pattern such as an optical disc substrate, a light guide plate, a micro lens array and a diffraction optical device.
The injection molding is a method in which a hot molten resin is injected into a mold and then is cooled. The injection compression molding is a method in which a cavity in the mold is set to be larger than a desired size and then is compressed to the desired size in the middle of a following injection. The injection compression molding has advantages that it can decrease an injection pressure to be applied to the resin and enhance a transcribability for a micro pattern.
The compression molding and the hot embossing molding are methods in which a plastic material of a powder, film, or block type is compressed with a mold heated to a predetermined temperature.
FIG. 1 illustrates a conventional molding system for the injection molding or injection compression molding and FIG. 2 is illustrates another conventional molding system for the compression molding or a hot embossing molding.
A mold insert 6 having a micro/nano pattern 8 on it is fixed onto a front side of a mold. (Here, the front side of the mold is a side facing a molding material and a back side of the mold is an opposite side thereto.) The mold includes a stationary part 2 and a moving part 4.
In the injection molding or injection compression molding, a cooling channel 10 is formed in the mold. Instead, a heating plate can be fixed onto the outer surface of the mold. The mold is heated to a desired temperature with a hot fluid heated to a predetermined temperature, and then a molten resin is injected into a cavity 12 through a sprue 14.
In the compression molding or hot embossing molding, a heater 16 is fixed onto the backside of the mold. The heater 16 heats the whole of the mold.
However, in the above molding systems, a quick heating and cooling is difficult and thus there are many problems. For example, in the injection molding or injection compression molding, as soon as a resin with high temperature is injected into the cavity of the mold and contacts with the mold, a solidified layer is created due to the temperature difference between the resin and the mold.
FIGS. 3A and 3B show that flow resistance increases and transcribability deteriorates in case of a center gate type (FIG. 3A) and a side gate type mold (FIG. 3B) due to growth of the solidified layer in the process of molding plastics using the conventional injection molding method or injection compression molding method.
Once the solidified layer is created, fluidity deteriorates and in result, it is difficult to completely transfer the micro/nano pattern. In addition, low fluidity makes the pressure applied onto the resein and the residual stress increase, which deteriorates mechanical and optical performance of products.
If a length of the cavity is large in comparison with a thickness thereof, these problems become worse and a more serious defect is caused. For example, in connection with an optical disc substrate, now it is technically possible to manufacture a DVD (Digital Versatile Disc) substrate with a diameter of 120 mm and a thickness of 0.6 mm satisfying diverse property requirements. However, if the size is smaller, that is, if the thickness is smaller than 0.6 mm, a complete filling during the injection molding is difficult due to the solidified layer. Even if a complete filling is attained, properties of the substrate become worse due to the residual stress. These problems are not only for the optical disc substrate but also for other high-performance plastic goods with a micro pattern.
In the compression molding and hot embossing molding, a conventional molding system has a disadvantage that because the whole of the mold has to be heated/cooled, a heating/cooling time is long and effectiveness of heating/cooling is low.
To solve these problems, heating means in contact with a front side or a back side of a mold insert which heats the mold insert are suggested. These heating means are disclosed, for example, in U.S. Pat. Nos. 6,276,656, 5,569,474, and 5,176,839.
FIG. 4 illustrates another conventional molding system as disclosed in the 656' patent.
A heater 147 is in the vicinity adjacent to the periphery of a cavity 110 and in thermal contact with a back side of a mold insert 132. Reference numerals 114, 120 and 124 represent boosters. Reference numerals 112, 122 represent fluid passages. Reference numerals 118, 128 and 140 represent a moving part, a stationary part, and an entrance, respectively.
Compared with the molding system in FIG. 1 and FIG. 2, molding systems in which the mold contacts with the front or back side of the mold insert have a advantage that the mold insert can be quickly heated and thus a response speed is very fast. Therefore, it is possible to improve transcribability and mechanical and optical properties of products. However, these molding systems have disadvantages below.
In the molding systems as disclosed in the 474' patent and the 839' patent, the heater is formed on a front side of a mold insert. This deteriorates transcribability for a micro pattern on the mold insert. Therefore, these molding systems are not suitable to form a micro pattern structure.
In the injection molding, injection compression molding, compression molding, and hot embossing molding, temperature is an important parameter. We can improve the quality of the products by detecting the temperature and using it in the molding process. In the molding system as disclosed in the 656' patent, the 474' patent and the 839' patent, the heater is conventionally attached or coated onto the mold insert. The above patents do not refer to a temperature sensor. However, if a temperature sensor has to be used as a component part of the molding system, it will be fixed onto the mold insert. That is, the temperature sensor as well as the heater is fixed onto the mold insert by a conventional way.
This conventional way requires much processing time and effort. Furthermore, it is very difficult to form a micro heater and a micro sensor at an exact position by the conventional way. Especially, if the plurality of the micro heaters and the plurality of the micro temperature sensors have to be formed, it is almost impossible to form them at exact positions by the conventional way.
More importantly, the temperature controllability is still bad in these molding systems. We know from a theory, experience or experiment that the temperature is not uniform throughout a molding material in the middle of the molding process. This non-uniformity of local temperatures means that the solidification of the molding material proceeds at different speeds. The different solidification speeds cause the residual stress to be produced inside of the products. In result, a deformation of the products is caused and optical properties of the products deteriorate.
These problems have a very serious effect on the molding process for a structure with a micro pattern on it, such as an optical disc substrate, a light guide plate, a micro lens array and a diffraction optical device, because a defect, even if very slight, has a very serious effect on the micro pattern structure in proportion to its precision.
Therefore, in the injection molding, injection compression molding, compression molding and hot embossing molding, especially for forming the micro pattern structure, a local temperature control is required.